The present invention relates to printing of thick emulsions through a stencil in general, and particularly to stencil printing of solder paste for electronics. Conventional screen printing as used for manufacturing electronic hybrid devices and surface mount technology favors flexible screens constructed from stainless steel screen, woven in an open mesh and stretched taut across a rigid rectangular frame. Openings are selectively provided in the screen in a pattern that represents the pattern of the desired print, and the ink, emulsion or solder paste is forced through the openings by the hydraulic action of the squeegee moving across the stencil surface. The deposited thickness of the ink or paste is controlled by the thickness of the screen.
Another form for printing, illustrated in FIG. 1, involves the use of a metal stencil 10 fabricated from a thin sheet of metal 11 stretched taut across a rigid frame 12. The openings 13 in the stencil 10 are machined or etched in the desired pattern, and the ink or paste 14 is printed through the openings as in above. Again, the thickness of the deposited material 14 is controlled by the thickness of the metal stencil 10.
Use of conventional mesh screens for solder paste printing for surface mount technology is not practical because of the relatively large volume of material required to be deposited on each printed area. Large openings in the screen cause uneven tension distributions in the taut screen, creating poor quality printing. In addition, the very close spacing required by surface mount technologies also affects the tension in the mesh screen. For these reasons, a metal stencil of 0.008 to 0.010 inches thick is typically used in place of the mesh screen.
In order to obtain a print height of 0.008 inches, the solder paste must be screened at a high viscosity to prevent "slumping" after printing, but as the template has little or no flexibility, it is not possible to bring it into intimate contact with the surface of substrate 15, due to variations in the surface contours of the substrate. Around the critical edges of the closely spaced openings in the stencil, it is not unusual to find a gap of 0.002 inches or more. The downward pressure created by the action of the squeegee 17 on the stencil is not enough to locally deform the stencil into intimate contact with the substrate. Hydraulic pressure on the solder paste 14 causes some of the paste to be extruded sideways in to the gap between the stencil and the substrate. This occurs at each squeegee stroke, and progressively builds up a deposit on the lower surface of the template, around each opening, so that the printed areas extend into the separating spaces between solder pads. The printed pattern is then no longer acceptable and the machine must be halted while the template is cleaned. Because the gap between stencil and substrate varies due to individual variations in the substrates, the frequency of cleaning also varies widely, and is a major obstacle to high speed automated printing.